02_constructor.md

9. Constructor names:

WARNING: In solc <0.4.22, constructor names had to be the same name as the contract containing it.

• Misnaming the constructor would make it a regular function (not a constructor).

WARNING: solc ^0.4.22 introduced the constructor keyword. Contracts could have both old-style and new-style constructor names,

• with the first defined constructor would taking precedence over the second if both existed, which also led to security issues.

RESOLUTION: Solc 0.5.0 forced the use of the constructor keyword.

Multiple constructors could be defined for solc ^0.4.22, using both the old and new format

contract A {
    uint x;
    constructor() public {
        x = 0;
    }
    function A() public {
        x = 1;
    }

    function test() public returns(uint) {
        return x;
    }
}

see here see here

10. Void constructor:

WARNING: Calls to unimplemented base contract constructors leads to misplaced assumptions.

BEST PRACTICE: Check if the constructor is implemented, or remove call if not.

The constructor of contract B calls the unimplemented constructor of A

contract A{}
contract B is A{
    constructor() public A(){}
}

see here

11. Implicit constructor callValue check:

BUG SUMMARY: The creation code of a contract that does not define a constructor() but has a base that does, did not revert for calls with non-zero callValue when such a constructor was not explicitly payable.

This is due to a compiler bug introduced in v0.4.5 and fixed in v0.6.8.

DESCRIPTION:

• Starting from Solidity 0.4.5 the creation code of contracts without explicit payable constructor is supposed to contain a callvalue check that results in contract creation reverting, if non-zero value is passed.
• However, this check was missing if no explicit constructor was defined in a contract at all, but the contract has a base that does define a constructor.
• In these cases it is possible to send value in a contract creation transaction or using inline assembly without revert, even though the creation code is supposed to be non-payable.

see here